1. Field of the Invention
The present invention is directed to a method for phase correction of nuclear magnetic resonance signals of the type wherein image echos are acquired under readout gradients of alternating operational sign, and are entered row-by-row into a raw data matrix.
2. Description of the Prior Art
In a magnetic resonance imaging technique known as echo planar imaging (EPI), nuclear magnetic resonance signals are generated in rapid sequence as gradient echos in the readout phase by continuously changing of the polarity of a readout gradient. Due to the changing polarity of the readout gradient, the samples acquired from the nuclear magnetic resonance signals must be entered into a raw data matrix such that the direction of entry changes from row to row of the raw data matrix. If even slight deviations occur from row to row, this leads to an artifact known as N/2 ghosts, i.e. the actual image, given an image matrix of N.times.N points, is imaged again but shifted by N/2 in the positive and negative directions with reference to the middle of the image matrix, these ghosts generally arising in the image with a different intensity than the "true" image.
Several methods are known for solving this problem. U.S. Pat. No. 5,138,259 discloses a method wherein an adjustment scan is implemented before the actual measurement. This adjustment scan differs from the measurement only in that it is implemented without phase coding. A complete correction data matrix is acquired from which various inadequacies of the measuring system can be identified and correction data can be calculated for the actual measurement, however, the overall measuring time is noticeably lengthened with this method.
U.S. Pat. Nos. 4,644,279 and 4,970,457 disclose a technique wherein a nuclear magnetic resonance signal is acquired before the actual measurement, with at least one of the magnetic field gradients being shut off during this acquisition. This nuclear magnetic resonance signal is used for determining the basic magnetic field. Image disturbances that are produced by variations of the basic magnetic field can thus be eliminated.
Further, the publication Proceedings of the Society of Magnetic Resonance in Medicine, Vol. 3, 12.sup.th Annual Scientific Meeting, 14-20 Aug. 1993, page 1239, discloses a technique wherein a phase coding pulse is omitted in the middle of the k-space. Two rows that are employed as reference values and which are acquired in a different direction of the k-space are thus obtained. After a Fourier transformation, relative time shifts are determined from the reference row in a two-stage method. The image data are corrected on the basis of the correction data acquired in this way. The method for acquiring the correction data, however, is rather complicated and a discontinuity in the data acquisition that can have a negative influence on the image quality arises due to the omission of a phase coding step.
IEEE Transactions on Medical Imaging, Vol. MI-6, No. 1, March 1987, pp. 32-36, discloses a phase correction method that, however, is not referred to the initially explained problem in the EPI method. Phase errors of the first order are thereby estimated by autocorrelation of the complex phase distortions of the image data, whereas a correction factor of the 0.sup.th order is acquired from the histogram of the phase distribution of the image corrected with respect to the first order.
U.S. Pat. No. 5,581,184 discloses a pulse sequence wherein nuclear magnetic resonance signals are acquired successively under readout gradients with alternating operation signs. A navigator echo (S.sub.1.sup.+, S.sub.2.sup.-) is thereby respectively measured without phase coding under a positive sub-pulse and under a negative sub-pulse of the readout gradient. Correction data are then produced on the basis of these two navigator echos. It has been proven, however, that this known method leads only to incomplete correction data, i.e. N/2 ghosts are not completely eliminated.